At the moment people give away the purchase history for next to nothing with loyalty cards and such.
Worse still, your so called Doctor knows more about your history than youself. The rightful way you may grand him/her access to your history for some time. Your data, your work, your comment... is your income, your money.
For something to be lawful it has to work both ways. If someone ask of proof of anything, ask for proof first, if satisfied may proceed to next level.
Can Banks & Governments 'Prove Their Wealth'?
If your doctor isn't doctor, but just "so called doctor", i think you should have a second opinion.
Some questions:
1. First of all, where do you live? GDPR is protecting your privacy in EU pretty well, or at least it's better then anywhere else.
2. Why would your doctor or "doctor" (what ever that is) would know more about your history then yourself. You are literally living your own history, so aren't you paying attention to it?
3. Why would he know anything about your income or money if you didn't give that information?
4. Proofs for fractional reserves for example are done via 3rd party audits. If you really want proof, contact the firms doing the auditing. But i am not sure what any firm would even gain from complying with some random dude's demands for tailored public access of what ever their vision of "proof" is.